To decrease corona loss. They are grouped because they are on the same phase not because being close together makes them carry the same voltage. Multiple lines on the same phase has less loss due to being able to use multiple smaller lines instead of one larger line. Electric field strength is partly determined by the gauge of the line. The larger the gauge the lower the field strength and therefore the more loss due to ionization. Smaller lines have a higher field strength and therefore less power loss due to ionization.